Thin film storage electrode



Sept. 21, 1965 H. J. HANNAM 3,

THIN FILM STORAGE ELECTRODE Filed May 19, 1960 f 6 FIG.I.

FIG.2.

FORM VAPORIZABLE SUPPORT FILM ACROSS ANNULAR SUPPORT MEMBER.

FlG.5.

FORM VAPDRIZABLE SUPPORT FILM ACROSS ANNULAR SUPPORT MEMBER.

DEPOSIT THIN LAYER OF ALUMINUM ON'VAPOR- IZABLE SUPPORT FILM.

DEPOSIT THIN LAYER OF NOBLE METAL ON VAPOR- IZABLE SUPPORT FILM.

DEPOSIT THIN LAYER OF MAGNESIUM ON ALUMINUM LAYER.

DEPOSIT THIN LAYER OF MAGNESIUM ON NOBLE METAL LAYER.

HEAT IN OXIDIZING AT- MOSPHERE TO DECOM- POSE VAPORIZABLE FILM OXIDIZEMETALLIC LAYERS AND EFFECT FUSION THEREOF.

HEAT IN OXIDIZING AT- MOSPHERE TO DECOM- POSE VAPORIZABLE FILM,OXIDIZEMAGNESIUM AND COAGULATE NOBLE METAL TO FORM DISCRETE ISLETS THEREOF.

INVENTORi HERBERT J. HANNAM,

HIS ATTORNEY.

United States Patent 3,207,937 THIN FILM STQRAGE ELECTRODE Herbert J.Hannam, Guilderland, N.Y., assignor to General Electric Company, acorporation of New York Filed May 19, 1960, Ser. No. 30,153 12 Claims.(Cl. 313-89) My invention relates to improved thin film storage targetelectrodes of the type for use in producing a point-bypoint electriccharge pattern corresponding to a visual image or other information tobe converted to electrical signals by scanning a target electrode withan electron beam. More particularly, my invention relates to an improvedthin-film target electrode and improved methods of manufacturing same.

In my US. Patent 2,922,907, issued January 26, 1960, and assigned to thesame assignee as the present invention, there is disclosed and claimed athin-film storage target structure comprising an annular support memberand an extremely thin taut low mass storage membrane of homogeneouspolycrystalline magnesium oxide extending across the annular supportmember and supported solely at its periphery by said support member.This target is adapted for extremely high sensitivity, improvedresolution and high resonant frequencies for avoiding undesirablemechanical vibrations and resultant unwanted electrical signalmodulations.

In manufacturing the above-described type of target electrode, it isdesirable to increase the tension of the membrane to raise the resonantfrequency thereof to desirable high amounts and to reduce the amplitudeof vibration to a desirable low amount. Additionally, it is desirable toreduce graininess of the membrane material to avoid detection of thegrains in images transmitted with such targets.

The present invention contemplates improved target electrodes andimproved methods of manufacturing same which will afford thin filmtarget structures adapted for all of the above-noted desirableproperties of my prior device and further adapted for substantiallyincreased membrane tension, reduction in graininess with resultantimproved electrical characteristics, and greater ease of manufacture.

Accordingly, a primary object of my invention is to provide new andimproved target electrode assemblies including improved target membranestherein.

Another object of my invention is to provide new and improved targetstructures including new and improved composite thin-film targetmembranes adapted for improved electrical characteristics.

Another object of my invention is to provide new and improved targetstructures including new and improved thin-film target electrodesadapted for increased resonant frequencies, reduced amplitude ofvibration and reduced membrane graininess.

Another object of my invention is to provide new and improved methodsfor manufacturing thin-film targets.

Another object of my invention is to provide new and improved storagetarget electrodes and improved methods of manufacturing same, wherebymanufacturing shrinkage is reduced substantially.

Further objects and advantages of my invention will become apparent asthe following description proceeds and the features of novelty whichcharacterize my invention will be pointed outwith particularity in theclaims annexed to and forming part of this specification.

In carrying out the objects of my invention, I provide a targetelectrode including an annular support member corresponding in diameteror transverse dimension generally to the diameter of a mesh electrodeusually used with a target electrode. Extending tautly across the anicenular electrode support and supported solely thereby is a compositemembrane comprising a first layer of a homogeneous polycrystallinemagnesium oxide and a second layer of a material selected from the groupincluding homogeneous polycrystalline aluminum oxide and the noblemetals. The target electrode can be manufactured by first forming avaporizable support film on an annular support member, depositing analuminum barrier on the support film, then depositing a magnesiumcoating on the aluminum, and then heating the assembly in an oxidizingatmosphere to decompose the vaporizable support film and to convert thealuminum to homogeneous polycrystalline aluminum oxide and the magnesiumto homo geneous polycrystalline magnesium oxide, thereby to leave a tautcomposite polycrystalline aluminum oxide and magnesium oxide membranesupported solely by the annular support member. Alternatively, thetarget electrode can be manufactured by first forming a vaporizablesupport film on an annular support member, depositing noble metalbarrier on the support film, then depositing a magnesium coating on thenoble metal layer, and then heating the assembly in an oxidizingatmosphere to decompose the vaporizable support film, to convert themagnesium to homogeneous polycrystalline magnesium oxide and tocoagulate the noble metal to form a layer of a myriad of discrete isletsof noble metal fused to the surface of the magnesium oxide.

For a better understanding of my invention reference may be had to theaccompanying drawing in which:

FIG. 1 is an enlarged sectional view of a storage target constructed inaccordance with one embodiment of my invention and wherein thethicknesses of the various layers of material are shown exaggeratedlyfor ease of illustration;

FIG. 2 is a flow chart illustrating the steps in one method ofmanufacturing a target electrode according to my invention;

FIG. 3 is an enlarged sectional view of a target structure illustratingthe structure at a particular point in the method of manufacture;

FIG. 4 is an enlarged sectional view of a storage target electrodeconstructed in accordance with a modified form of my invention; and

FIG. 5 is a flow chart illustrating the steps involved in another methodof manufacturing a target electrode according to my invention.

Referring to the drawings, there is shown in FIGURE 1 a storage targetstructure generally designated 1 and constructed according to oneembodiment of my invention. The target 1 includes a first annularsupport member 2 to the upper surface of which is suitably secured asecond annular support member or ring electrode 3. If desired thesemembers may be integral and constitute a unitary element.

Extending across the ring 3 and supported solely thereby is a compositetransparent semi-conductive membrane generally designated 4. Themembrane 4 includes a first layer 5 of a homogeneous polycrystallineoxide which is semi-conductive and adapted for substantially straightthrough grain boundary electron conduction and a superposed fused secondlayer 6 of a material which is adapted for maintaining the desiredthrough conductivity of the oxide layer and for lower secondary electronemissivity than the mentioned oxide layer.

Preferably the membrane 4 is constructed such that the first layer 5consists of interconnected granules of homogeneous polycrystallinemagnesium oxide and the second layer 6 consists of interconnectedgranules of homogeneous polycrystalline aluminum oxide fused by areaction product to the granules of the magnesium oxide layer. Themagnesium oxide granules are formed of magnesium oxide crystallites ofapproximately 300 angstroms. The overall thickness of the compositemembrane 4 is between approximately 500 and approximately 1000 angstromsand preferably about 750 angstroms. Additionally, the thickness of thesecond or aluminum oxide layer is between approximately 0.5% andapproximately 10% of the overall thickness of the membrane. Preferablythe thickness of the second layer is between approximately 1% andapproximately 3% of the overall thickness of the membrane.

The target structure 1 is adapted for being incorporated in a targetelectrode assembly of the type disclosed and claimed in my above-notedpatent which includes an electron-permeable mesh supported in closelyspaced parallel relation with the magnesium oxide layer, and theassembly is adapted for use in a camera tube structure of the typeillustrated in my mentioned patent, for example an image orthicon cameratube. In such a device the magnesium oxide surface is exposed to thephotocathode in the head or front end of the tube and the aluminum oxidesurface is exposed to the electron beam usually emanating from theopposite end of the tube. In the target electrode illustrated, themembrane 4 is extremely taut and the exposed surface of the magnesiumoxide layer 5 is fine grained and substantially free of mottling withthe grains being up to approximately only microns. This fine graininessreduces the presence of substantially large grain boundaries in themembrane which are subject to greater secondary emission and thus canundesirably appear on a transmitted image. The aluminum oxide layer 6 isalso adapted for through electron conductivity and, thus, does notsubtract from the desirable electrical characteristics of the magnesiumoxide. Additionally, the aluminum oxide is adapted for less secondaryelectron emission than magnesium oxide and, thus, is better suited forexposure to impinging electrons from the beam and also serves tominimize the appearance of grain boundaries in a transmitted image.

In accordance with one method of constructing the target electrode ofFIGURE 1, and as outlined in the chart shown in FIGURE 2, a suitablethin vaporizable film which can advantageously be nitrocellulose isformed across the annular support member 3 as illustrated at 7 in FIGURE3. The vaporizable film 7 can be formed by first dropping onto thesurface of a pan of water a small quantity of nitrocellulose dissolvedin a suitable organic solvent such as amyl acetate. This solutionspreads on the surface of the water to a thin film due to surfacetension and the solvent evaporates, leaving a plastic film on thesurface of the water. Thereafter, the membrane support ring 3 which hasbeen placed in the water either prior to formation of the film or whichis immersed in the water at the outer portion of the film, is raisedgently to pick up the film on the surface of the ring.

After the film has been dried completely on the ring, the ring is placedin an evaporator and a thin coating of aluminum shown at 8 in FIGURE 3is evaporated on the plastic film to a thickness up to thatcorresponding to approximately 2% to 35% optical opacity.

Subsequently, a magnesium coating 9 is evaporated on the aluminumcoating. The thickness of the magnesium thus evaporated on the aluminumis determined by the desired mechanical and electrical characteristicsof the target electrode and is controlled to provide the desired throughelectron conductivity and proportions of the finished oxide layersdescribed above.

Thereafter, the structure is placed in an oven and heated in anoxidizing atmosphere which can be air, starting at a temperature ofapproximately 170 centigrade and terminating at a temperature of about430 centigrade with the heating continuing for a period in the order ofapproximately 5 hours. This baking step serves to decompose and vaporizethe nitrocellulose film which disappears completely and is alsoeffective for converting the aluminum to homogeneous polycrystallinealuminum oxide and compounds in the film.

the magnesium to homogeneous polycrystalline magnesium oxide and forfusing the resultant layers of interconnected crystalline oxide granulesto provide the abovedescribed composite self-sustaining homogeneouspolycrystalline oxide membrane adapted for being supported solely at itsperiphery by the support ring. Additionally, the membrane is ofsubstantially greater tautness and the grain size of the magnesium oxideconstituting the layer thereof is uniformly substantially finer thanthat obtained with the prior method involving depositing the magnesiumdirectly on the vaporizable film during the formation process.

As presently understood, the increased tightness of the membrane and thedesirable fineness of the grain structure of the magnesium oxide is dueto several aspects of my invention. First, the deposition of themetallic aluminum on the plastic film before deposition of the magnesiumis believed to provide a smoother, more planar and a more water-freesurface upon which the evaporating magnesium lands during depositionthereof. These conditions of the surface upon which the magnesium isdeposited are believed to contribute substantially to the increasedtightness of the membrane and the smoother, more fine-grained texture ofthe magnesium oxide layer obtainable with my method. Additionally,aluminum does not react with nitrocellulose as readily as magnesium.Thus, it is believed that the metallic aluminum layer deposited on thevaporizable film for the evaporization thereon of metallic magnesiumserves as a barrier between the magnesium and the vaporizable filmmaterial, thereby to avoid reactions between the hot magnesium and waterin the thin film or the breakdown of organic It is believed that suchreactions of hot magnesium and water and breakdown of the film materialcan have the undesirable effect of increased grain size and reduction ofthe tension of magnesium oxide. It is also believed that these reactionsmay occur during subsequent processing of the electron structure as whenmagnesium is oxidized by baking in an oxidizing atmosphere.

The aluminum layer serves as an effective barrier during the evaporationof the magnesium and during subsequent processing to avoid theundesirable reactions with the water-carrying film and decompositionproducts of the vaporizable film material and thus serves to retardgrain growth for improving the grain structure and to afford a tightermembrane. Additionally, the use of alu minum is highly desirable becausewhen converted to an oxide it too is adapted for high lateral resistanceand high straight through electron conductivity comparable to that ofthe magnesium oxide. Thus, the aluminum oxide is ineffective forelectrically shunting discrete points of the magnesium oxide layer andthus does not subtract from the desired electrical characteristics ofthe magnesium oxide layer. Additionally, and as pointed out above, thealuminum oxide layer is characterized by lower secondary emissivity thanthe magnesium oxide and thus is better adapted for use on the electronbeam-impinged side of the membrane.

Illustrated in FIGURE 4 is a modified form of my improved targetelectrode generally designated 10. This structure is also adapted forthe same applications as the structure disclosed and claimed in myabove-noted patent. Additionally, the structure also includes first andsecond annular support members 11 and 12, respectively. Extending acrossthe member 12 is a composite storage membrane generally designated 13including a semi-conductive layer 14 and a layer comprising a myriad ofdiscrete islets 15 of non-oxidizing conductive material such as a noblemetal, for example gold fused to one surface of the layer 14. The layer14 is constituted of interconnected granules of a homogeneouspolycrystalline oxide which is self-sustaining in that it is adapted forbeing supported solely at its periphery. Additionally, it issemi-conducting and adapted for substantially straight through electronconduction. The islets 15, by being discrete for forming a discontinuousconductive surface are rendered ineffective for electrically shuntingthe discrete points on the layer 14 and thus do not subtract from thedesired through electron conductivity of the oxide layer. Additionally,the islets 15 are formed of .a material having a lower secondaryemissivity than the oxide layer.

Preferably, the structure in FIGURE 4 includes a first layer 14 ofhomogeneous polycrystalline magnesium oxide and a fused second layer ofa noble metal subdivided into a myriad of discrete islets orcoagulations. The overall thickness of the composite membrane 13 isbetween approximately 500 and approximately 1000 angstroms andpreferably about 750 angstroms. Additionally, the thickness of the layerof islets 15 is between approximately 1% and and preferablyapproximately 5% of the overall thickness of the membrane. In thisembodiment of my invention also, the membrane 13 is extremely taut andthe magnesium oxide layer 14 is substantially free of mottling and isfine-grained in that the grain size is up to approximately only 10microns. This fine graininess also reduces substantially the presence oflarge grain boundaries in the membrane which would be subject to greatersecondary emission and thus could appear on a transmitted image.Additionally, in this structure the noble metal is a lower secondaryemitter than the magnesium oxide and thus is better adapted forimpingement by the electron beam. As pointed out above, the fact thatthe noble metal is broken up into a myriad of discrete mutually spacedand thus electrically isolated islets renders the noble metalineffective for electrically shunting discrete points on the first layerwhich would adversely affect the electrical characteristics of the oxidelayer.

Outlined in FIGURE 5 is the method whereby I con struct the targetelectrode of FIGURE 4. This method includes a step of forming avaporizable film across the annular support member 12 which can becarried out in the same manner as that described above in connectionwith the construction of the electrode of FIGURE 1. After thevaporizable film has been formed and completely dried the support isplaced in an evaporator and a layer of from approximately 10 toapproximately 100 angstroms of a noble metal such as gold is depositedon the vaporizable film. Thereafter, a magnesium coating is evaporatedon the noble metal layer. The thickness of the magnesium thus depositedis determined by the desired through electron conductivity and themechan ical and electrical characteristics of the target electrode andis controlled to provide the desired properties of the finishedcomposite membrane structure described above. At this point of theprocess of manufacture the unfinished structure has an arrangement oflayers of material and an appearance comparable to that shown in FIGURE3.

Subsequently, the assembly is subjected to the same heating step asdescribed above in connection with the structure in FIGURE 1. This hasthe desirable effect of decomposing and vaporizing the plastic supportfilm and converting the magnesium to a homogeneous polycrystallinemagnesium oxide which is self-sustaining in that it requires onlyperipheral support as provided by the annular support member 3. Duringthe heating process the noble metal serves as a barrier between themagnesium and nitrocellulose in the same manner as the aluminum in thefirst-described method, thereby to provide a taut membrane having afine-grained magnesium oxide surface and for providing a low secondaryemission surface facing the reading beam in a camera tube.

At the elevated temperature and after the noble metal has served itsfunction as a barrier means, the noble metal coagulates to form adiscontinuous film or a layer of a myriad of discrete islets 15 on andfused to the surface of the magnesium oxide. The discontinuous nature ofthis film, or the electrical isolation of the discrete islets, insuresagainst adverse effects on the through conductivity of the magnesiumoxide in the manner described above. Additionally, the islets 15 havelow secondary emissivity relative to that of the magnesium oxide whichis desirable for a surface exposed to beam impingement.

In addition to providing target electrodes of increased membranetightness and improved surface textures my ZlbOVC-dfiSCIlbBdIIICthOdSfacilitate and reduce the cost of manufacture. Specifically,nitrocellulose films of lower grades than previously required can beused in view of the fact that the evaporating magnesium in the presentmethods do not see or become deposited on the nitrocellulose and, thus,costly adverse effects by the nitrocellulose on the magnesium oxide arereduced substantially. Additionally, the magnesium evaporation rate canbe slowed down considerably during the deposition step due to the factthat the interposed metal barrier or layer prevents the evaporatingmagnesium from liberating gas from the plastic support member.

While I have shown and described specific embodiments of my invention Ido not desire my invention to be limited to the particular forms shownand described; and I intend by the appended claims to cover allmodifications within the spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A storage electrode comprising a taut composite storage membrane,said membrane consisting of a first and exposed layer of a homogeneouspolycrystalline magnesium oxide and a second layer of a materialselected from the group consisting of homogeneous polycrystallinealuminum oxide and the noble metals, said second layer being fused tosaid first layer and formed to be substantially ineffective forelectrically shunting discrete points on said first layer.

2. A storage electrode according to claim 1, wherein the exposed surfaceof said magnesium oxide layer is constituted of oxide crystallites ofapproximately 300 angstroms and having a grain size up to approximatelyonly 10 microns.

3. A storage electrode comprising an annular support member, a compositestorage membrane supported solely at the periphery thereof by saidsupport member and having a thickness of approximately 500 toapproximately 1000 angstroms, said membrane consisting of a first layerof a homogeneous polycrystalline semiconducting oxide characterized bysubstantially straight through electron conductivity and high lateralresistance and a second layer of a material characterized by highlateral resistance fused to said first layer.

4. A storage electrode comprising an annular support member, a compositestorage membrane supported solely at the periphery thereof by saidsupport member, said membrane comprising a first layer of homogeneouspolycrystalline magnesium oxide, and a second layer of material selectedfrom the group consisting of homogeneous polycrystalline aluminum oxideand the noble metals said second layer being fused to said first layerand characterized by high lateral electrical resistance along saidlayer.

5. A storage electrode comprising a composite storage membrane having anoverall thickness of approximately 500 to approximately 1000 angstroms,said membrane consisting of a first layer of magnesium oxide, a secondlayer of aluminum oxide fused to said first layer, and said second layerconstituting between approximately 0.5% and approximately 10% of saidoverall thickness.

6. A storage electrode according to claim 5, wherein said magnesiumoxide is constituted of crystallites of approximately 300 angstroms andhaving a grain size up to approximately only 10 microns.

7. A storage electrode comprising an annular support member, a compositestorage membrane extending tautly across and solely supported by saidsupport member, said membrane having an overall thickness ofapproximately 500 to approximately 1000 angstroms, said membraneconsisting of a first layer of homogeneous polycrystalline magnesiumoxide, a second layer of homogeneous polycrystalline aluminum oxidefused to said first layer, and said second layer constituting betweenapproximately 0.5% and approximately 10% of said overall thickness.

8. A storage electrode comprising a composite storage membrane having anoverall thickness of approximately 500 to approximately 1000 angstroms,and said membrane consisting of a first layer of magnesium oxide, and asecond layer of a noble metal sub-divided into an electricallydiscontinuous myriad of discrete islets fused to said first layer.

9. A storage electrode according to claim 8, wherein the thickness ofsaid second layer constitutes between approximately 1% and 10% of theoverall thickness of said membrane.

10. A storage electrode according to claim 8, wherein said magnesiumoxide layer is constituted of crystallites of approximately 300angstroms and having a grain size up to approximately only 10 microns.

1-1. A storage electrode comprising an annular support member acomposite membrane extending tautly across and solely supported by saidsupport member, said membrane having an overall thickness ofapproximately 500 to approximately 1000 angstroms, said membraneconsisting of a first layer of homogeneous polycrystalline magnesiumoxide, and a second layer of noble metal fused to said first layer andsub-divided into an electrically dis continuous myriad of discreteelectrically isolated islets.

12. The invention as recited in claim 1 wherein said noble metal is goldand which defines a layer of electrically discontinuous discreteisolated islets.

References Cited by the Examiner UNITED STATES PATENTS 2,527,652 10/50Pierce 313-89 X 2,527,732 10/50 Graham 313-89 X 2,822,493 2/58 Harsh313-89 2,922,907 1/60 Hannam 313-68 2,923,843 2/60 Turk 313-68 2,947,6518/60 Toohig 117-222 3,002,124 9/61 Schneeberger 313-68 X 20 GEORGE N.WESTBY, Primary Examiner.

RALPH G. NILSON, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,207,937 September 21, 1965 Herbert J Hannam It is hereby certifiedthat error appears in the above numbered patent requiring correction andthat the said Letters Patent should read as corrected below.

Column 8, line 13, for "2,527,732" read 2,527,632

Signed and sealed this 3rd day of May 1966.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner ofPatents

1. A STORAGE ELECTRODE COMPRISING A TAUT COMPOSITE STORAGE MEMBRANE,SAID MEMBRANE CONSISTING OF A FIRST AND EXPOSED LAYER OF A HOMOGENEOUSPOLYCRYSTALLINE MAGNESIUM OXIDE AND A SECOND LAYER OF A MATERIALSELECTED FROM THE GROUP CONSISTING OF HOMOGENEOUS POLYCRYSTALLINEALUMINUM OXIDE AND THE NOBLE METALS, SAID SECOND LAYER BEING FUSED TOSAID FIRST LAYER AND FORMED TO BE SUBSTANTIALLY INEFFECTIVE FORELECTRICALLY SHUNTING DISCRETE POINTS ON SAID FIRST LAYER.